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Myopia is a large public health concern both in the United States and worldwide. Previous research has shown that time spent outdoors is protective against the development of myopia. Intrinsically photosensitive retinal ganglion cells (ipRGCs) encode ambient light levels and help regulate a variety of functions including pupil constriction. The purpose of the study was to examine different methods of measuring time outdoors and to examine the association between exposure to outdoor light, spherical equivalent refractive error, and presumed ipRGC input to the pupil responses to red and blue light. Subjects were 20 young adults (mean age 24.9±1.8 years, 13 female) who wore a personal visible light monitor (Daysimeter) and an ultra-violet light monitor on an armband for an average of 7.1 ± 0.43 days. Time outdoors was measured by both the Daysimeter and UV badge, as well as a survey completed at a follow up visit. For the purposes of the study, time outdoors was defined as exposure to >1000 lux or any non-zero UV value. Light exposure over the previous 1, 3, 12, and 24 hours, 3 days, and 5 days was calculated as was total light exposure in log(lux-minutes). Using the RAPDx, pupilometer, pupil responses to 0.1Hz flashes were tested under 3 stimulus conditions: 1) alternating red and blue, 2) red-only, and 3) blue-only. The alternating trial lasted for 2 minutes and the single color trials lasted 1 minute with 5 minutes of dark adaptation prior to each trial. The study showed that the Daysimeter was a good method for the measurement of time outdoors because it agreed best with the activity survey (slope = 0.68, R2 = 0.46). Pupil constriction to red tended to increase during the trial when alternated with blue. There were significant associations between this increasing pupil constriction to red when alternated with blue for longer-term, greater light exposure (r = 0.46, p = 0.042 at 3 days; r = -0.51, p = 0.021 at 5 days) but not for the shorter time periods (1, 3, 12, and 24 hours). The difference in blue pulses during the alternating color protocol and blue pulses during the single color protocol were significantly related to spherical equivalent refractive error (r = 0.58, p = 0.008).The Daysimeter was validated in the study as a good objective measurement for time outdoors. The association between light exposure over several days and differences in the pupil response suggests that there is an ocular pathway that is modified by long-term light adaptation. The interaction between preceding blue light pulses and red light-driven pupil constriction is consistent with a role for blue-light-sensitive ipRGCs mediating this adaptive effect. Greater photopotentiation shown by larger differences in alternating and single color blue values were associated with less myopic refractive error. This is consistent with the theory that time outdoors is beneficial due to greater exposure to visible light and therefore an increase in ipRGC activity.